1. Field of the Invention
The present invention relates to a technique for manufacturing master discs of optical discs., and more specifically, to a technique for manufacturing master discs of high-density optical discs using electron beams.
2. Description of the Background Art
An electron beam recorder exposes and records signal patterns by focusing and irradiating electron beams on a master disc with resist applied. When the resist master disc with signal patterns exposed is developed, a master disc with concavo-convex shapes such as signal pits, and guiding grooves used for tracking at the time of record/reproduction can be obtained.
FIG. 10 is a schematic representation of conventional electron beam recorder 900. Electron beam recorder 900 includes electron beam source 901 that discharges electron beams, acceleration electrode 902 that accelerates the electron beams discharged from electron beam source 901, electron beam lens 903 that converges accelerated electron beam 912, and electrode 904 that varies a travel direction of electron beam 912 according to information signals. When an information signal is provided to a voltage controller (not illustrated), the voltage controller applies voltage to electrode 904 and deflects the electron beams. Electron beam recorder 900 further includes shield plate 905 that shields irradiation of the electron beams to resist master disc 907 when the electron beams are deflected, electron beam lens 906 that converges the electron beams onto resist master disc 907, resist master disc 907 with resist applied, turntable 908 that rotates resist master disc 907, and slider 909 that slides rotating turntable 908 in a radial direction of resist master disc 907. Each component of electron beam recorder 900 is arranged in a vacuum. Electron beam recorder 900 slides resist master disc 907 in the radial direction by slider 909 while rotating turntable 908, and exposes, i.e. records, signal patterns in the form of a spiral from the inner circumferential side of the circular resist master disc to the outer circumferential side or from the outer to the inner.
FIG. 11 shows a diagram of the positional relationship between shield plate 905 and the electron beams as viewed from the electron beam source 901 (FIG. 10). In the figure, resist master disc 907 is shown together. When voltage is not applied to electrode 904 (FIG. 10), the electron beams pass a position 921. Consequently, resist master disc 907 are irradiated with the electron beams. Because resist master disc 907 rotates in the direction designated by arrow mark 923, so-called pits are formed during irradiation of the electron beams. When a predetermined voltage is applied to electrode 904 (FIG. 10), the electron beams move to the direction of being shielded by shield plate 905 along passage 920 in the recording tangential direction and reach position 922. Therefore, in resist master disc 907, one or more areas which are not pit, that is, space, are formed. By moving the electron beams to position 921 or 922 along passage 920, desired patterns are formed on resist master disc 907. Edges of shield plate 905 are arranged to come in contact with the electron beams when the electron beams pass position 921.
FIGS. 12A through 12C explain specific examples of recording desired pit patterns. FIG. 12A shows a signal that forms a recording mark when voltage is of a high level and a space between pits when voltage is of a low level. The voltage controller (not illustrated) of electron beam recorder 900 (FIG. 10) generates modulating signals shown in FIG. 12B from the signal shown in FIG. 12A. When the voltage controller enters modulating signals into electrode 904 (FIG. 10), the electron beams are veered in a recording tangential direction based on the signals. To be more specific, when the modulating signal is 0V, the electron beams are applied to resist master disc 907 without being bent, and as modulating signals become negative voltages, the electron beams are veered in a direction of shield plate 905 (FIG. 11). When the modulating signal shown in FIG. 12B is entered into electrode 904 (FIG. 10), the electron beams make a round trip between positions 921 and 922 (FIG. 11) along passage 920 (FIG. 11), and pits are recorded. FIG. 12C shows the recorded pits.
In general, when the pit width to be recorded or width of guiding groove is changed, electron beam recorder 900 changes the rotating linear velocity of turntable 908 (FIG. 10) and adjusts an energy volume of the electron beams irradiated per unit area for resist master disc 907 (FIG. 1). This is because electron beam irradiation volume or electron beam acceleration voltage are unable to be varied at a high velocity. However, a setting of electron beam acceleration voltage can be varied. Varying the electron beam acceleration voltage can achieve the same effects obtained by varying laser beam wavelength when an optical disc is exposed using laser beams. Table 1 below shows relationships between electron beam acceleration voltage and converted laser beam wavelength. As clear from the table, as the acceleration voltage is increased, the converted wavelength is shortened. Consequently, the beam is able to be focused and therefore, optical discs can be highly densified.
TABLE 1Acceleration voltage (KV)510152025Converted wavelength (pm)248124836250
However, in conventional electron beam recorders, it is difficult to realize high densification of discs. This is because when patterns are recorded with a conventional electron beam recorder, a tear-drop shape pit with widths of leading end and the trailing end varied is formed as shown in FIG. 12C, and a uniform-shape pit with the uniform shape of pit leading end and trailing end cannot be obtained. Under such conditions, when the minimum pit length is still more shortened as still higher densification takes place, there is a high possibility of causing read/write errors.
The reasons why uniform pits are unable to be formed include the following. Because first of all, in electron beam recorder 900 (FIG. 10), the electron beams make round trips between shield plate 905 and resist master disc 907, the electron beams scan the recording tangential direction only shown in passage 920 (FIG. 11). Consequently, with respect to the resist master disc rotating direction, the electron beams move onto resist master disc 907 in the forward direction at the start of recording and move in the reverse direction from resist master disc 907 to shield plate 905 in the end of pit recording. The energy volume of the electron beams per unit area applied to the resist master disc is large at the start of pit recording (that is, when the pit leading end is formed) and is small in the end of recording when the pit trailing end is formed). Consequently, the width of pit leading end is wide and that of pit trailing end becomes narrow.